The invention concerns a solid state nuclear magnetic resonance (NMR) method for investigating a sample material which contains protons H and also spin-1/2 hetero nuclei X, wherein the sample material is rotated at the magic angle (MAS=magic angle spinning), the method comprising the following steps:                a) increasing the equilibrium polarization of X;        b) eliminating (2) proton magnetization;        c) transferring polarization from X to H;        d) recording the proton signals (8) under a condition of line narrowing;wherein the sequence a) through d) is repeated several times.        
A method of this type is disclosed by Y. Ishii, J. P. Yesinowski and R. Tycko, J. Am. Chem. Soc. 2001, 123, 2921-2922.
Nuclear magnetic resonance (NMR) spectroscopy has become one of the most powerful methods of analytical chemistry in the last decades. Elements with non-vanishing nuclear spin can be excited in an external magnetic field such that their magnetic moments precess. The associated precession frequency (resonance frequency) can be determined through irradiation and absorption of electromagnetic waves and depends on the environment of the analyzed nucleus. The NMR measurement therefore yields information about the binding partners of the nuclei examined and the binding separations between neighboring nuclei.
Numerous specific methods are used to increase the accuracy of NMR measurements, e.g. rotation of solid state samples at the magic angle.
X. Zhao, J. L. Sudmeier, W. W. Bachovchin and M. H. Levitt, J. Am. Chem. Soc. 2001, 123, 11097-11098 have proposed a method for precisely determining the binding separations between hydrogen and abundant amounts of a hetero nucleus having a nuclear spin 1/2 in the sample material. The method is based on hetero nuclear dipolar re-coupling using an R1825 sequence. This method was used for 15N—1H groups with artificially enriched i.e. increased 15N content. It is not suitable for hetero nuclei in small concentrations, e.g. 15N in natural concentrations in a sample material.
The work by Ishii et al. 2001 loc. cit. presents investigations of 13C in PMMA plastic materials. The method used therein is based on the indirect detection of 13C magnetization via the hydrogen spectrum. A polarization transfer from H to 13C, elimination of H magnetization through two radio frequency (RF) pulses under a rotation resonance recoupling condition, and a qualitative transfer of polarization from 13C back to H obtained pure correlation spectra of chemical shifts. Precise determination of binding separations is not possible with the method according to Ishii et al. 2001.
In contrast thereto, it is the underlying purpose of the present invention to provide a method for precise determination of binding separations between protons and a hetero nucleus with nuclear spin 1/2 which produces good results even for hetero nuclei concentrations, which are small, compared to that of protons, and which requires only a few hours of measuring time.